1. Field of the Invention
The present invention relates to a method for obtaining high purity titanium.
2. Discussion of the Background
Due to the rapid increase in the degree of large-scale integration (LSI) in recent years, electrode materials are undergoing a transition to those materials with higher purity and strength. For example, due to the present demand for a remedy to the signal delay caused by thinner electrode wiring, the focus is now being placed on metal materials with lower resistance, higher purity, and higher melting point, compared to the frequently used polysilicon. Metal materials with the above properties which are usable as electrodes in LSI include molybdenum, tungsten, titanium and their silicides. Among these, titanium is particularly promising, because of its excellent specific strength, workability and corrosion resistance.
In order to be used as electrode material in semiconductors, titanium metal must be of high purity. A typical method for obtaining high purity titanium is the iodide thermal decomposition process (also known as the iodine process). A conventional iodide thermal decomposition process will be described in conjunction with FIG. 3.
While a deposition substrate 23 is held at the axial center of a reactor 22 housed inside an electric furnace 21, crude titanium 24 is held inside the reactor 22, surrounding the deposition substrate 23. In this state, after evacuating the inside of the reactor 22 by use of a pump 28, iodine in an iodine container 26 is led into the reactor 22. Titanium deposition is then initiated by heating the deposition substrate 23 by passing through it an electric current from a power supply 25. Inside the reactor 22, the following reactions (1) and (2) take place. ##STR1##
The reaction of crude titanium 24 with iodine to form TiI.sub.4 proceeds on the perimeter of the reactor 22, on which the crude titanium is held at reaction temperatures of 200.degree.-400.degree. C. The thermal decomposition reaction of titanium tetraiodide proceeds on the deposition substrate 23 at the axial center of the reactor 22, depositing high purity titanium on the deposition substrate 23. The reaction temperature of the thermal decomposition reaction is 1300.degree.-1500.degree. C. The iodine produced by the thermal decomposition of titanium tetraiodide diffuses to the perimeter of the reactor 22, to be recycled for reaction with crude titanium 24.
As the deposition substrate 23, a high purity titanium filament with a diameter of 0.1-2 mm is normally used, but some attempts have been made to use plate shaped deposition substrates (Published Unexamined Patent Application No. Sho 62-294175 and No. Hei 2-73925). The crude titanium 24 used is typically in the form of a Ti sponge or machining chip in its particulate agglomerate state, and is housed in a molybdenum net 27, to be held inside the reactor 22. The reactor 22 is typically made of quartz or metal and is often lined with molybdenum to prevent gas corrosion by iodine or titanium iodides at high temperatures.
Purifying titanium by the conventional iodide thermal decomposition process has the following three problems:
The first problem is decreased productivity resulting from the use of a filament as the deposition substrate. Thus when a filament with a diameter of 0.1-2 mm OD is used as the deposition substrate, the rate of deposition is slow due to the small surface area of the deposition substrate at the initial stages of the reaction, and thus the productivity of high purity Ti is low. The filament can only be heated by generating resistance to an electric current passed through the filament. Since electrical resistance undergoes change as the filament diameter increases during the reaction, ensuring overall temperature control and maintenance of a uniform temperature over the deposition area is difficult. A localized low filament temperature might cause etching or wire-disconnection, particularly where filament and electrode leads are connected. Conversely, parts which are locally heated are susceptible to wire-disconnection from fusion.
If plate shaped deposition substrates are used, the surface area of the deposition substrate is higher initially, thus overcoming the disadvantage of low productivity. However, as in the case of the filament deposition substrate, the plate shaped deposition substrate can only be heated electrically. This is primarily due to the inability to transfer heat generated by a heater to the deposition substrate when the deposition substrate is located at the axial center of the reactor. Once again the problem of maintaining adequate temperature control during electrical heating remains unresolved.
The second problem concerns the use of crude titanium as the raw material in the process. Crude titanium, in the form of sponge or machining chips, is used in its particulate agglomerate state. However, crude titanium does not hold its shape well when charged into the reactor. Therefore, it is secured using a net made of a corrosion-resistant metal, such as molybdenum. However, this net is typically weakly secured and susceptible to break-up, making it difficult to charge large quantities of Ti into the reactor, and limiting scale-up of the apparatus.
The third problem involves the reactor. Quartz or metals, such as stainless steel, inconel, and Hastelloy, have previously been used as reactor materials. Typically a molybdenum lining is applied on the inside surface of the reactor to prevent gas corrosion by iodine or titanium iodides. While molybdenum has excellent corrosion resistance, after powder-sintering it is fragile and susceptible to cracking when assembling or dismantling the reactor, thus detracting from its repetitive use.